The present invention relates to a lithium secondary battery which is excellent in the charging discharging cycle characteristics, and has high reliability, and is suitably used particularly as a battery for driving a motor of an electric vehicle.
In recent years, the lithium secondary battery is being rapidly and widely used to realize a small power source for portable electronic equipment. In addition, effort of development is being also made to realize practical use of the lithium secondary battery as a motor driving battery for an electric vehicle which replaces a gasoline-powered vehicle, and as a battery for storing electric power in the night.
In the lithium secondary battery, a lithium transition metal compound oxide such as lithium-cobalt oxide (LiCoO.sub.2), lithium-nickel oxide (LiNiO.sub.2), or lithiummanganese oxide (LiMn.sub.2 O.sub.4) is used as a positive active material, while various carbon materials are used as a negative active material. At charging, lithium ions in the positive active material are transferred to the negative active material. Contrariwise, at discharging, lithium ions captured by the negative electrode are transferred to the positive electrode. Thus, charging and discharging are performed.
The structure of lithium secondary battery is roughly divided into a wound type and a laminated type. Of them, the wound type is constituted by inserting an internal electrode body 1, which is formed by winding a positive electrode 2 and a negative electrode 3, as shown in FIG. 4, through a separator 4, into a tubular container, and suitable for producing a compact battery while using electrodes with large area. In the case of this wound type, since it is sufficient that there is at least one lead for current collection 5 from each electrode 2, 3, and, even if it is desired to lower electricity collection resistance of each electrode 2, 3, it is sufficient to increase the number of leads, there is an advantage that the internal structure of battery does not become complicated to make easy assembly of the battery.
Here, noting the charging/discharging mechanism again, when the lithium ions are transferred from the positive electrode to the negative electrode at charging, LiCoO.sub.2 or the like as the positive active material causes cubical expansion as the lithium ions are desorbed. On the other hand, the negative active material is expanded as it captures the lithium ions desorbed from the positive active material. For example, if graphite is used as the negative active material, it is confirmed that spacing is separated as lithium ions are intercalated between atomic layers of graphite. Therefore, in the lithium secondary battery, both the positive and negative electrodes would expand at charging.
On the contrary, at discharge where the lithium ions are transferred from the negative electrode to the positive electrode, both the positive and negative electrodes would contract. It has been found that such expansion/contraction of electrodes is more significant in the negative electrode than the positive electrode. Moreover, it has been found that the charging/discharging electrode shows a larger change in its volume when the same carbon material with a high degree of graphitization is used as the negative active material. Therefore, although it is particularly desirable to use a material with a low degree of graphitization to suppress volume change of the negative electrode, a material with a higher degree of graphitization is preferable to reduce the size of battery and to improve volume and weight energy density since it has higher specific gravity, and a ratio of lithium ions contributing to charging/discharging which can be retained per unit weight is high (smaller amount of dead lithium).
In the wound-type internal electrode body, substantially constant static pressure (tightening pressure) is applied to each electrode since each electrode is wound under a substantially equal force when it is produced. However, as described above, since a volume change of expansion/contraction occurs in each electrode at charge/discharge, repetition stress would be caused in both the positive and negative electrodes and the separator in the winding direction. As this stress breaks the tightening pressure on the internal electrode body, there may arise peeling of the electrode active material, partial bending or generation of cracks in the electrode, and cyclic degradation of compressing/breakdown of the separator. Moreover, this degradation may cause internal pressure rise caused by local heating and evaporation of electrolyte from partial concentration of current and/or abnormal discharge caused by internal short-circuiting. This stress will be larger if the winding length of the electrodes is longer.
Such cyclic degradation is undesirable for the battery characteristics regardless of its application. However, the cyclic degradation particularly tends to occur in a battery with a large capacity of 50 Wh or more which is required for a battery for an electric vehicle (EV) or a hybrid electric vehicle (HEV) since such a battery has a total length of several meters for the positive and negative electrodes in the winding direction. Such cyclic degradation leads to lowering a of running performance. In addition, there is a possibility that an accident involving the battery caused by abnormal current generated from the cyclic degradation would lead to an unfathomable severe accident compared with a small battery.